Sequence Determinants for Protein Import into Chloroplasts and Thylakoid Membrane Protein Assembly

Abstract

Most nuclear encoded chloroplast proteins are targeted to the organelle by transient N-terminal extensions or transit peptides (cTPs). Transit peptides from higher plants are rich in hydroxylated residues but contain only few acidic residues; in contrast cTPS from Chlamydomonas contain many positively charged residues, and appear to be able to form amphiphilic α-helices. Upon import, cTPs are cleaved by a stromal processing peptidase. In many but not all cTPs the cleavage site is characterized by a weakly conserved motif: (I/V)-X-(A/C) ↓ A. Proteins destined for the lumen of the thylakoids have a second targeting signal placed C-terminally to the stromal transit peptide that has a structure very similar to the secretory signal peptides of prokaryotes. Finally, integral thylakoid membrane proteins have recently been shown to have a preponderance of positively charged residues in their stromafacing domains, and thus follow the same “positive inside-rule” that characterises both prokaryotic and eukkryotic plasma membrane proteins. It thus seems likely that the processes of translocation across, or integration into, the thylakoid membrane are mechanistically similar to the corresponding processes in bacteria.